As a variable displacement type hydraulic pumps, swash plate type axial piston pumps are generally known. Hydraulic pumps of this type can vary their discharging capacity by regulating the inclination angle of a swash plate (swash plate angle).
FIG. 1 schematically shows a displacement varying mechanism for a conventional swash plate type axial piston pump 1. The displacement varying mechanism has a control cylinder 2, which is disposed in the vicinity of one side of the swash plate 3 of the hydraulic pump 1, and its piston rod 4 is pivotally secured to the end of that side of the swash plate 3. When the pilot pressure of the discharge side of the hydraulic pump 1 is applied to the control cylinder 2, the piston rod 4 of the control cylinder 2 is telescopically extended and contracted to vary the inclination angle .theta. of the swash plate 3. The pilot pressure of the hydraulic pump 1 to the control cylinder 2 is controlled by a solenoid switching valve 5 interposed in a pilot conduit 6, and the switching valve 5 has its positions switched by a signal from a switching valve driving circuit 7.
In a conventional hydraulic pump 1 having the displacement varying mechanism as described above, the pilot pressure of the discharge side of the pump 1 is zero when the pump 1 is stopped, and the swash plate angle .theta. is maximized by a compression spring 8 connected to the other side of the swash plate 3. Accordingly, the discharge capacity of the pump 1 is at maximum when the pump is started so the driving load of the hydraulic pump 1 becomes maximum, and thus the load applied to a drive source 9 for driving the hydraulic pump 1 becomes large. Particularly, in the case of industrial vehicles, since the hydraulic pump 1 is used as a drive source for loading/unloading operations, etc., it should have a relatively large discharge capacity, and the load applied to the drive source 9 is inevitably large. Therefore, if the viscosity of hydraulic fluid or working oil is high at low temperatures, there is a fear that the startability of the industrial vehicle will be deteriorated.